Pilots generally rely on instrument landing systems (ILSs), and/or distance measuring equipment (DME) to aid with navigation and landing when flying during periods of low visibility or inclement weather.
The ILS is a ground-based instrument approach system that provides aircraft with lateral guidance (e.g., from localizer antenna array) and vertical guidance (e.g., glide slope antenna array) while approaching and landing on a runway. In principle, an aircraft approaching a runway is guided by ILS receivers in the aircraft that perform modulation depth comparisons of signals transmitted by a localizer antenna array located at the end of the runway and by a glide slope antenna array located to one side of the runway touchdown zone. In addition to the lateral and vertical navigation information the Localizer identifies the ILS system by transmitting a unique Morse code identifier. The ILS signal format also includes a provision for a voice signal that is not currently in use. When a DME is included as part of an ILS, it also provides a Morse Code identification that is the same as that provided by the associated Localizer. The Morse Code signals must be time synchronized to assure that only the Localizer or the DME identification is broadcast at any one time.
For navigation, two signals are transmitted by the localizer from co-located antennas within the array. One signal is modulated at a first frequency (e.g., 90 Hz), while the other signal is modulated at a second frequency (e.g., 150 Hz). Each of the co-located antennas transmits a narrow beam, one slightly to the left of the runway centerline, the other slightly to the right of the runway centerline. The localizer receiver in the aircraft measures the difference in the depth of modulation (DDM) of the first signal (e.g., 90 Hz) and the second signal (e.g., 150 Hz). The depth of modulation for each of the modulating frequencies is 20 percent when the receiver is on the centerline. The difference between the two signals varies depending on the deviation of the approaching aircraft from the centerline. The pilot controls the aircraft so that a localizer indicator (e.g., cross hairs) in the aircraft remains centered on the display to provide lateral guidance. The same transmitter also transmits a Morse Code identification signal. This allows the pilot to positively identify the ILS system. The depth of modulation for the Morse Code is between 5% and 15%.
Similarly, the glide slope (GS) antenna array transmits a first signal modulated at a first frequency (e.g., 90 Hz) and a second signal modulated at a second frequency (e.g., 150 Hz). The two GS signals are transmitted from co-located antennas in the GS antenna array. The center of the GS signal is arranged to define a glide path of a predetermined slope (e.g., 3°) above the ground level for the approach of the aircraft. The pilot controls the aircraft so that a guide slope indicator (e.g., cross hairs) remains centered on the display to provide vertical guidance during landing. The Glide slope does not have a provision for Morse Code transmission.
Navigational aid systems may include an ILS that is made of a localizer, a glide slope, and DME. As discussed supra, the localizer provides horizontal guidance and the glide slope provides vertical guidance to an aircraft approaching a runway. The DME may be collocated with the localizer or the glide slope. The localizer, glide slope, and DME may be identified by an aircraft using synchronized identification signals broadcast from the Localizer and DME. The localizer may broadcast respective identifications using a frequency of the voice band. In conventional navigation aid systems, the broadcast of the identification signal of the DME is triggered using a synchronization signal received from the localizer using a cable connection. For example, when the DME is located remote from the localizer, a cable that may be several miles long may be used to connect the DME to the localizer and/or glide slope. However, this configuration may be inconvenient and expensive.
Therefore, there exists an unmet need in the art for methods, apparatuses, and computer-readable media to trigger a synchronized broadcast of the DME identification without using an expensive cable connection.